Nonlinear effects in optical fibers limit the distance over which non-repeatered or non-regenerated lightwave communications can occur. Nonlinear effects such as stimulated Brillouin scattering and stimulated Raman scattering manifest themselves as power-dependent excess fiber losses which limit the transmission distance between repeaters. Another type of nonlinear limitation occurs in longhaul fiber systems in which the signal pulses are boosted by optical amplifiers placed at more or less uniform intervals along the transmission path. The total length of such a system is limited by the cumulative nonlinear phase shifts imposed on the pulse envelopes by the optical nonlinearity of the transmission fibers. The peaks of the pulses, where the optical power is largest, are repeatedly phase-shifted relative to the tails of the pulses, where power is low. For a system length large enough to allow the relative nonlinear phase shifts to approach .pi./2, the pulse distortions are too large to permit recovery of the signal with a low error rate.
The magnitude of such nonlinear effects is proportional to the optical power density of a signal and is thus inversely proportional to the effective cross-sectional area A.sub.eff for nonlinear effects in an optical fiber (hereinafter referred to as the effective area of the fiber). Thus, a larger effective area permits longer fiber spans between repeaters and longer systems, which result in more cost effective and more capable communication systems. Certain commercially-available fibers have a relatively large effective area.
Fiber dispersion is another optical characteristic of a fiber which impacts optical transmission. Fiber dispersion causes spreading of the frequency spectrum of typical non-return-to-zero (NRZ) pulses transmitted through the fiber. As a result, the signal spectrum can broaden cumulatively along a fiber transmission system. Since pre-receiver filtering must have a bandwidth large enough to allow detection of the important frequency components of the signal, larger dispersion requires larger filtering bandwidth. As a result, more of the broadband noise from the optical amplifiers in the transmission path must be admitted into the receiver, with a consequent degradation of signal-to-noise ratio and increased error rates. Thus, it is desirable to achieve zero fiber dispersion at the operating wavelength (typically near 1550 nanometers) of NRZ transmission systems. If soliton pulses are used to transmit information, the useful dispersion tends to be relatively small and positive, but not zero. Certain commercially available fibers, known as dispersion-shifted fibers, achieve zero-dispersion at wavelengths near 1550 nanometers. Other commercially available fibers have zero-dispersion wavelengths near 1310 nanometers, and very large dispersion near 1550 nanometers.
While there are some fibers having large effective areas and other fibers having minimal dispersion at the desired wavelengths, there are no commercially available optical fibers which, at the operating wavelength, have both a desirable fiber dispersion characteristic and a relatively large effective area. Commercial fibers that have a relatively large effective area typically exhibit poor fiber dispersion characteristics at the operating wavelength whereas other commercial fibers that have desirable fiber dispersion characteristics at the operating wavelength typically exhibit a relatively small effective area. Usually, a system design specifies one type of fiber thereby incurring the penalty of either high dispersion or small effective area.